Chemical looping combustion method using dual metal compound oxide

ABSTRACT

A chemical looping combustion method using a dual metal compound oxide includes the following steps: a fuel material combusting with the dual metal compound oxide in a first reactor to obtain a metal product; supplying the metal product obtained in the first reactor into a second reactor, and the metal product reacting with the air in the second reactor to obtain the dual metal compound oxide; and, supplying the dual metal compound oxide obtained in the second reactor into the first reactor. The dual metal compound oxide used in the chemical looping combustion process has high oxidation rate as well as high reduction rate so as to increase the efficiency of the chemical looping combustion process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a chemical looping combustion method usingdual metal compound oxide, and more particularly, to the chemicallooping combustion method capable of increasing reaction rate anddecreasing the difficulty in design for the chemical looping combustion.

2. Description of the Prior Art

In recent years, science and technology were developed rapidly, however,the overexploitation of resource and the pollutant in the development ofscience and technology cause a huge impact to the environment to forcepeople taking the environmental protecting issue seriously. Thedevelopment of science and technology is highly related to the energyresources, therefore, the impact caused by power plants to theenvironment and the development of green energy are significant parts inthe environmental protecting issue.

The thermal power generating method is the most general power generatingmeans in the world. The power is generated by combusting fossil fuelssuch as coal, gasoline, or gas to heat water and produce steam to pushthe power generator. Compared to other power generating methods, thethermal power generating method produces more air pollution. Because ofthe combustion of the fossil fuels with the oxygen in the air in thereactor, the exhaust gas produced in the thermal power generating methodincludes carbon dioxide. Carbon dioxide is generally considered as oneof greenhouse gases, and the decrement of carbon dioxide emissions is anobjective in many countries. Besides the decrement of carbon dioxideemissions, the carbon dioxide storage and reuse also reduce theemissions of carbon dioxide. However, the exhaust gases includepollutants such as nitrogen oxide, and the pollutant must he separatedbefore the storage and reuse of carbon dioxide. The separation of thepollutant from the exhaust gases consumes much energy, and a part of thepower generated by the power plant is applied to the separation process,in other words, the power generating efficiency of the thermal powerplant is lowered down.

In the prior art, the chemical looping combustion process taking metaloxygen carriers, which is used for replacing air, as comburent has beendisclosed to solve the above-mentioned problem. The chemical loopingcombustion process uses two fluidized bed reactors, the fuel reactor andthe air reactor, to process oxidation reaction and reduction reactionrespectively to generate heat. In detail, the fuel materials combustwith the metal oxygen carriers to release heat, and the combustionreaction of the fuel materials means a reduction reaction of the metaloxygen carriers, so that the metal oxygen carriers are reduced to metalafter the combustion. Then, the reduced metal is supplied into the airreactor and oxidized with air or other gas capable of providing oxygen.After the oxidation of the reduced metal in the air reactor, the metaloxygen carriers is generated from the reduced metal and then supplied tothe fuel reactor to keep looping.

Compared to the conventional combustion reactions, the chemical loopingcombustion process provides oxygen by the metal oxygen carriers insteadof the air, so that the exhaust gases generated in the chemical loopingcombustion process includes 99% carbon dioxide after removing the watervapor via condensation. The high-purity carbon dioxide can be stored andreused directly and the separation process of the other pollutants fromthe exhaust gases which consumes high energy is not necessary.Accordingly, the chemical looping combustion process is beneficial tothe decrement of carbon dioxide emissions of the thermal power plant andavoids the energy consumption on separating the gas, that is to say, itis more efficient on power generation.

The ferrous oxygen carrier and the nickel oxygen carrier were often usedas the metal oxygen carriers in the chemical looping combustion process.The ferrous oxygen carrier has high oxidation rate but low reductionrate. The nickel oxygen carrier has high reduction rate but lowoxidation rate. The mismatch of reaction time results in a difficulty indesign for the chemical looping combustion process, and it is harmful tothe application of the chemical looping combustion process to thethermal power plant.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a novelchemical looping combustion method to solve the problem in the priorart.

According to an embodiment of the invention, a chemical loopingcombustion method includes the following steps of: supplying a dualmetal compound oxide into a first reactor; a fuel material combustingwith the dual metal compound oxide in the first reactor to obtain ametal product and a gas; supplying the metal product obtained in thefirst reactor into the second reactor; the metal product reacting withthe air in the second reactor to obtain the dual metal compound oxide;and, supplying the dual metal compound oxide obtained in the secondreactor into the first reactor. The dual metal compound oxide includesan oxide of a first metal, an oxide of a second metal, and an oxide of acompound of the first metal and the second metal.

In this embodiment, the dual metal compound oxide is prepared by thefollowing steps of: mixing a metal salt of the first metal and a metalsalt of the second metal uniformly in an aqueous solution to obtain adual metal solution; adding a water-soluble polymer into the dual metalsolution to obtain a precipitate, and then drying the precipitate; and,pulverizing and calcining the dried precipitate to obtain the dual metalcompound oxide.

On the advantages and the spirit of the invention, it can be understoodfurther by the following invention descriptions and attached drawings.

BRIEF DESCRIPTION OF THE APPENDED DRAWINGS

FIG. 1 is a flow chart illustrating a chemical looping combustion methodaccording to an embodiment of the present invention.

FIG. 2 is a flow chart illustrating the chemical looping combustionmethod according to another embodiment of the present invention.

FIG. 3 is a flow chart illustrating the method for preparing the dualmetal compound oxide according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Please refer to FIG. 1. FIG 1 is a flow chart illustrating a chemicallooping combustion method according to an embodiment of the presentinvention.

As shown in FIG. 1, the chemical looping combustion method of theembodiment includes the following steps of: in step S10, supplying adual metal compound oxide into a first reactor; in step S12, a fuelmaterial combusting with the dual metal compound oxide in the firstreactor to obtain a metal product, a gas, and heat; in step S14,supplying the metal product obtained by the combustion in the firstreactor into a second reactor; in step S16, the metal product reactingwith the air in the second reactor to obtain the dual metal compoundoxide; and, in step S18, supplying the dual metal compound oxideobtained in the second reactor into the first reactor.

In this embodiment, the dual metal compound oxide in step S10 could be acompound including an oxide of a first metal, an oxide of a secondmetal, and an oxide of a compound of the first metal chemical bondingwith the second metal. For example, if the first metal is iron and thesecond metal is nickel, the dual metal compound oxide could be a Fe—Nicompound oxide including Fe₂O₃, NiO, and NiFe₂O₄.

In step S12, the fuel material could be fossil fuel, such as methane orpropane, generating combustion reaction with the dual metal compoundoxide to obtain the metal product, the gas, and heat. For example, acombustion reaction of methane with Fe—Ni compound oxide produces iron,nickel, a compound of iron and nickel, gas, and heat. The reaction inthe first reactor is a reduction reaction for the dual metal compoundoxide. To ensure the combustion of fuel material with the dual metalcompound oxide but not with other oxygen sources, an atmosphere withoutoxygen can be filled in the first reactor to avoid the reaction of thefuel material with other oxygen sources. In practice, the first reactorcan he filled up with an inert gas first, and then the combustionreaction proceeds.

The heat generated in step S12 is used to heating water o produce steamwhich can drive the power generator. Because the fuel material is fossilfuel such as methane or propane in step S12, the gas obtained includescarbon dioxide and water vapor. According to another embodiment, the gasobtained in step S12 can be processed by the following steps of: drawingthe gas from the first reactor; condensing the gas to remove the watervapor from the gas; and, sealing and storing the condensed gas. Thewater vapor is removed from the gas, so that the condensed gas is highpurity carbon dioxide and can be sealed and stored directly. Besides,the condensed gas can be reused directly to reach the purpose ofdecrement of the carbon dioxide emissions.

Please refer to FIG. 1 again. In the embodiment, the first reactor andthe second reactor can he fluidized bed reactors, and the metal productcan be supplied into the second reactor (air reactor) directly when itis obtained in the first reactor (combustion reactor), as described instep S14. And then, in step S14, a suitable condition is provided to thesecond reactor to assist the metal product to be oxidized therein toobtain the dual metal compound oxide. For example, the above-mentionedmetal product including iron, nickel and the compound of iron and nickelcan be oxidized to obtain a compound oxide including Fe₂O₃, NiO, andNiFe₂O₄, i.e., the dual metal compound oxide reacting with the fuelmaterial in step S12. The reaction of the metal product in the secondreactor is an oxidation reaction, and the air or an oxygenic atmospherecan be filled in the second reactor.

As in step S18, the dual metal compound oxide obtained in the secondreactor is supplied into the first reactor. In practice, the dual metalcompound oxide can react with the fuel material to obtain the metalproduct, the gas, and heat again if the supply of the fuel material andother conditions in the first reactor is sufficient. Please refer toFIG. 2. FIG. 2 is a flow chart illustrating the chemical loopingcombustion method according to another embodiment of the presentinvention. As shown in FIG. 2, after the dual metal compound oxideobtained in the second reactor in step S16 is supplied into the firstreactor as described in step S18, step S12 can be executed again to formloops for continuously generating heat. The other steps in thisembodiment are substantial the same with the corresponding steps in theabove-mentioned embodiments, so that the detail would not be describedagain.

In this embodiment, step S12 and step S16 are executed stage by stage,that is to say, after reducing the dual metal compound oxide, theproduct of the reduction is oxidized to obtain the dual metal compoundoxide. According to the ratio of the two metals in the dual metalcompound oxide, the reacting time of the reduction reaction and that ofthe oxidation reaction can be adjusted. Summarily, the dual metalcompound oxide for the chemical looping combustion method has highreduction rate and high oxidation rate, and then the reaction time ofthe reduction reaction is close to that of the oxidation reaction.Accordingly, the chemical looping combustion method of the presentinvention can be provided with high efficiency and easily designed.

Please refer to FIG. 3. FIG. 3 is a flow chart illustrating the methodfor preparing the dual metal compound oxide according to anotherembodiment of the present invention. The dual metal compound oxideprepared by the method in this embodiment can be the oxygen carrierprovided to the combustion of the fuel material in the above-mentionedembodiments. As shown in FIG. 3, the method for preparing the dual metalcompound oxide includes the following steps of: in step S20: mixing ametal salt of the first metal and a metal salt of the second metaluniformly in an aqueous solution to obtain a dual metal solution; instep S22, adding a water-soluble polymer into the dual metal solution toobtain a precipitate, and then drying the precipitate; finally, in stepS24, pulverizing and calcining the dried precipitate to obtain the dualmetal compound oxide.

Take Fe—Ni compound oxide for example. In step S20, a ferric salt and anickel salt are mixed in different ratio in the aqueous solution toobtain the Fe—Ni metal solution, wherein the ferric salt can be ferricnitrate and the nickel salt can be nickel nitrate. The water-solublepolymer, which is used to be the dispersing agent, such as polyethyleneglycol, is added into the Fe—Ni metal solution, and the precipitate isthen obtained and dried, as described in step S22.

In step S24, the dried precipitate is pulverized and then calcined. Whenthe calcining temperature reach a specific temperature range, the ironand nickel is oxidized to form Fe₂O₃ and NiO, and iron and nickel issintered and then oxidized to form NiFe₂O₄. Therefore, the Fe—Nicompound oxide includes the three oxides and the compound of the threeoxides. In practice, the temperature range to calcining driedprecipitate can be 500° C. to 1600° C., and preferably, can be 700° C.to 1100° C. It should be noted that the calcining temperature range canbe adjusted according to the kinds of the two metals.

Please refer to Table. 1. Table. 1 shows different conditions forpreparing the Fe—Ni compound oxides according to the above-mentionedembodiments. As shown in Table. 1, the samples 1˜4 are prepared bymixing nickel nitrate and ferric nitrate in different ratio in water,adding the polyethylene glycol selectively, and pulverizing andcalcining the precipitate at 700° C. or 1000° C. for 6 hours to obtainthe dual metal compound oxides. Please refer to Table. 2. Table. 2 showsthe oxidation time and the reduction time required by the chemicallooping combustion processes using the Fe—Ni compound oxides of thesample 1˜4, a pure ferric oxide, a pure nickel oxide, and a pure Fe—Nioxide (the oxide of the compound of iron and nickel).

TABLE 1 calcining polyethylene nickel ferric Sample H2O (mL) temperatureglycol nitrate nitrate 1 15 1000 3 10 30 2 15 700 0 30 10 3 15 700 0 3010 4 30 700 3 30 10

TABLE 2 Total reduction Total oxidation Sample time (mins) time (mins) 16.4 4.5 2 7.8 18.9 3 7.6 18.6 4 14.6 6.1 pure ferric oxide 89.5 1.3 purenickel oxide 3.7 >150.0 pure Fe—Ni oxide 83.0 13.0

As shown in Table. 1 and Table. 2, although the conditions for preparingthe Fe—Ni compound oxides in the samples 1˜4 are different and thereduction time and the oxidation time of the chemical looping combustionprocesses following the conditions are different too, the reduction timeand the oxidation time of the chemical looping combustion processesusing the Fe—Ni compound oxides of the samples 1˜4 are shorter andcloser to each other compared to those using pure ferric oxide, purenickel oxide, and pure Fe—Ni oxide. In other words, the Fe—Ni compoundoxides have high oxidation rate and high reduction rate at the same timefor the chemical looping combustion processes. Oppositely, the oxidationrates of the pure fenic oxide and pure Fe—Ni oxide are far larger thanthe reduction rates of those, but the reduction rate of the pure nickeloxide is far larger than the oxidation rate of that. The close oxidationtime and reduction time of the Fe—Ni compound oxides of the samples 1˜4for the chemical looping combustion processes are advantageous to thedesigns for the chemical looping combustion processes.

According to another embodiment, the dual metal compound oxide for thechemical looping combustion method can be loaded on a carrier, and thenthe dual metal compound oxide and the carrier can be supplied togetherinto the first reactor or the second reactor to be reduced or oxidized.For example, after preparing the Fe—Ni compound oxide by the methodshown in FIG. 3, the Fe—Ni compound oxide can be loaded on a carriermade of inert material. The Fe—Ni compound oxide and the carrier can besupplied into the first reactor by the method shown in FIG. 1 so that tothe fuel material combust with the Fe—Ni compound oxide on the carrier.It should be noted that the combustion reaction of the fuel material isthe reduction reaction of the Fe—Ni compound oxide. After the reductionreaction, the metal product is formed on the carrier. The metal productand the carrier is supplied into the second reactor to oxidize the metalproduct. It further raises the reactivity and decreases the operatingtemperature of the chemical looping combustion process by supplying thedual compound oxide loaded on the carrier.

To sum up, the chemical looping method of the present invention executesthe reduction reaction and the oxidation of the dual metal compoundoxide respectively in two reactors so as to keep looping to continuouslygenerate heat for the power generator. Compared to the prior art, thedual metal compound oxide in the present invention has high oxidationrate and high reduction rate at the same time so that the reaction timeof the two reactions in the chemical looping combustion process areshort and close to each other. Therefore, the problem of difficulty indesign for the chemical looping combustion process in the prior art canbe solved effectively, so that the chemical looping combustion processcan be easily used in the thermal power plant for the decrement of thecarbon dioxide emissions.

Although the present invention has been illustrated and described withreference to the preferred embodiment thereof, it should be understoodthat it is in no way limited to the details of such embodiment but iscapable of numerous modifications within the scope of the appendedclaims.

1. A chemical looping combustion method using dual metal compound oxide,comprising the following steps of: supplying a dual metal compound oxideinto a first reactor, wherein the dual metal compound oxide comprises anoxide of a first metal, an oxide of a second metal, and an oxide of acompound of the first metal and the second metal; a fuel materialcombusting with the dual metal compound oxide in the first reactor toobtain a metal product and a gas; supplying the metal product obtainedin the first reactor into a second reactor; the metal product reactingwith the air in the second reactor to obtain the dual metal compoundoxide; and supplying the dual metal compound oxide obtained in thesecond reactor into the first reactor.
 2. The method of claim 1, whereinthe dual metal compound oxide is a Fe—Ni compound oxide, and the metalproduct comprises iron, nickel, and a compound of iron and nickel. 3.The method of claim 2, wherein the Fe—Ni compound oxide comprises Fe₂O₃,NiO, and NiFe₂O₄.
 4. The method of claim 1, further comprising thefollowing steps of: mixing a metal salt of the first metal and a metalsalt of the second metal uniformly in an aqueous solution to obtain adual metal solution; adding a water-soluble polymer into the dual metalsolution to obtain a precipitate, and then drying the precipitate;pulverizing and calcining the dried precipitate to obtain the dual metalcompound oxide.
 5. The method of claim 1, wherein the temperature forcalcining dried precipitate is in the range of 500° C. to 1600° C., andpreferably 700° C. to 1100° C.
 6. The method of claim 4, wherein thedual metal compound oxide is a Fe—Ni compound oxide, the first metal isiron, the second metal is nickel, the metal salt of the first metal isferric nitrate, the metal salt of the second metal is nickel nitrate,and the water-soluble polymer is polyethylene glycol.
 7. The method ofclaim 11, wherein the fuel material is methane.
 8. The method of claim1, further comprising the following steps of: loading the dual metalcompound oxide on a carrier; and supplying the dual metal compound oxideand the carrier into the first reactor.
 9. The method of claim 1,wherein the first reactor and the second reactor are fluidized bedreactors.
 10. The method of claim 1, further comprising the followingsteps of: drawing the gas from the first reactor; condensing the gas toremove the water vapor from the gas; and sealing and storing thecondensed gas.